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Diagnose Exhaust Smoke

Copyright AA1Car

Engine smoke means trouble! An engine in good running condition should not produce any smoke in its exhaust. Steam is normal, and may appear to be white smoke on a cold morning. But any other type of smoke in the exhaust means something is wrong.

Possible Causes of Exhaust Smoke:

BLUE SMOKE: Bad news because it means the engine is burning oil. The underlying cause may be worn or broken piston rings, worn or damaged cylinders, worn valve guide or valve guide seals.

WHITE SMOKE: More bad news because it means the engine is burning coolant or transmission fluid. If the smoke is coolant, the cause if probably a leaky head gasket or a crack in the cylinder head. If the smoke is transmission fluid, the engine is sucking transmission fluid through a vacuum hose to the transmission.

BLACK SMOKE: Normal for older diesel engines when first started, but you should not see any visible black smoke with a gasoline engine. If you do, it means the air/fuel mixture is too rich possibly because of a stuck automatic choke on an older engine with a carburetor, a leaky fuel injector, too much fuel pressure, or clogged air filter.

Diagnosis of Exhust Smoke:

BLUE SMOKE: Usually smells like burned toast. Check the oil level on the dipstick to see if the oil is low (add oil as needed to bring it back up to the full mark. DO NOT let the engine oil level get too low or serious engine damage will result!. Do a compression check or leak down test to diagnose worn pistons or rings.

WHITE SMOKE: May have a slightly sweet smell if coolant, a burned smell if transmission fluid. Check the coolant level and the transmission fluid level. If the coolant is low and/or the engine has been overheating, pressure test the cooling system to see if it holds pressure. If it does not, the head gasket is probably leaking and needs to be replaced. If only the transmission fluid level is low, add the required type of transmission fluid to bring it back up to the full mark, and inspect the vacuum hose from the transmission for fluid inside. If it is passing fluid, replace the vacuum modulator valve on the transmission.

BLACK SMOKE: Check the automatic choke if the engine is an older one with a carburetor. Check fuel pressure if the engine is a newer one with fuel injection. Also inspect the air filter.

Repairs:

Repairs will depend on what is causing the smoke. If the engine is burning oil (blue smoke), it will probably need rings and valve guide seals, which means overhauling the engine. If the engine is burning coolant (white smoke), the cylinder head will have to come off to replace the head gasket. If the engine is blowing black smoke, adjusting the automatic choke on the carburetor, or replacing the leaky fuel injector or defective fuel pressure regulator, or a clogged air filter may be required.




Related Articles:

Causes of High Oil Consumption
Valve Guide Repairs
Engine Compression Testing
Engine Leakdown Testing
Piston Rings (steel)
Piston Rings (stock & performance)
Piston Ring End Gap Recommendations
Measuring Blowby
Why Head Gaskets Fail
Head Gasket Failure: Common Causes
Preventing Repeat Head Gasket Failures
How To Find & Fix Coolant Leaks
Understanding Fuel Delivery Systems
How To Diagnose & Repair Carburetor Problems
Air Filters

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